This invention relates to metal foundry techniques. In particular, this invention relates to techniques employed in metal founding to obtain a formed metal casting which requires a minimum amount of grinding or machining after being removed from the mold.
Typically, cast metal articles are made in the foundry industry by introducing molten metal into casting molds, and allowing the metal to cool and harden. Since the molten metal shrinks on cooling, resulting in an imperfect casting if further molten metal is not introduced to fill the voids in the mold created by shrinkage, the industry conventionally employs risers, or molten metal reservoirs to supply molten metal to the filled mold as required during the cooling process.
It is necessary that these risers be provided with an opening or port communicating with the mold cavity sufficiently large to prevent the formation in the riser of a constricting throat at this juncture that will act as a heat dissipator or sink. This occurrence will, of course, result in a blockage of the channel between riser and mold with cooled, hardened metal, and prevent the liquid metal in the riser from flowing into the mold cavity as required. Thus, risers are normally provided with ports of fairly large cross-sectional area.
The use of risers with such large port areas has, however, contributed its own problems. Although the absence of a constricting throat in these risers generally permits sufficient molten metal into the mold cavity during hardening to obviate the problem of shrinkage in the casting, the presence of the large cross-sectional area constitutes a strong connection between casting and riser when the liquid metal therein has cooled and hardened.
The riser is, of course, integrally cast with the casted article in the mold, and the riser must subsequently be separated from the casting. Such separation is frequently effected by, for example, sawing apart the two sections in the vicinity of the port between riser and casting; the casting must then be ground off to remove excess metal, and machined to obtain a smooth finish in the separation area.
The separation process is further complicated by the fact that often padding metal is included in the casting at the juncture of riser and casting; this excess metal is provided as a "cushion" between the casting and the separation point in order to aid in preventing rupture of the casting itself when it is forcibly separated from the riser. This padding metal must also be removed by grinding prior to machining. These separation procedures are time-consuming and expensive, especially in terms of labor and time costs.
Another problem associated with the use of risers and the introduction of molten metal therethrough into the mold cavity is that of metal flow control between the riser and mold cavity. Although it is frequently desirable to retain the molten metal within the riser for a period of time sufficient to permit the disposition of impurities such as dross or slag to reduce the impurity content of the molten metal flowing into the casting cavity, constrictions required to effect this at the riser port have at times resulted in the metallic blockage problem noted above.
This problem has been partially solved by the use of metal strainers to trap larger impurities in the molten metal flow channels when casting such metals as aluminum or magnesium; these strainers cannot, however, be used when casting steel, as the very high temperatures involved would result in the fusion of these strainers.
Accordingly, it is an object of this invention to provide a means of casting metal articles by a process using at least one riser whereby the riser may be readily separated from the cast metal article after cooling.
It is a further object of this invention to provide a means of casting metal articles by a process using at least one riser whereby the flow of molten metal through the riser port into the mold cavity is controlled, thereby permitting impurities such as dross or slag to be disposed within the riser away from the riser port.
It is an additional object of this invention to provide a means of casting metal articles by a process using at least one riser whereby no blockage of the channel between riser and mold cavity occurs, and yet separation of the riser from the cast metal article after cooling may be readily effected.
It is another object of this invention to provide a means of casting metal articles by a process using at least one riser whereby the cast metal article may be readily separated from the riser after cooling, and require minimum amounts of grinding and/or machining thereafter to remove excess metal.
It is yet another object of this invention to provide an inexpensive means, effective at high temperatures such as encountered in steel casting, whereby a metal casting comprising a riser portion and cast article portion may readily be separated into its component parts.
Further objects and uses of the invention will be apparent from the following specification and claims appended hereto.